The present invention disclosed in the specification relates to a method and a device of irradiating laser beam by scanning the laser beam which is fabricated in a linear shape.
The present invention disclosed in the specification can be utilized in exposure technology in laser annealing technology and photolithography steps in respect of, for example, a semiconductor.
In recent years, researches are carried out widely on the technology in which an amorphous semiconductor film or a crystalline semiconductor film (semiconductor film having crystallinity of not single crystal but polycrystal, microcrystal or the like) formed over an insulating substrate of glass or the like, that is, a non single crystal silicon film, is crystallized or the crystallinity is promoted by performing laser annealing. A silicon film is frequently used for the semiconductor film.
A glass substrate is inexpensive and rich in workability compared with a quartz substrate that has been conventionally used frequently, and is provided with an advantage of capable of easily fabricating a substrate having a large area. This is the reason for carrying out the above-described researches. Further, laser is preferably used for crystallization because in the laser process, a substrate is not heated and the process is suitable for using a glass substrate having low heat resistance.
A crystalline silicon film formed by performing laser annealing is provided with high mobility. When such a crystalline silicon film is used, TFTs (Thin Film Transistor) for driving pixel and for drive circuit can be integrated on one sheet of glass substrate.
Generally, the laser spot of laser beam is provided with dimensions of several centimeters or less, respectively, and therefore, a special device is needed in carrying out processing in respect of a large area.
Generally, pulse laser beam of excimer laser or the like is fabricated by an optical system such that a square spot of several centimeters square is formed and the laser beam is scanned (irradiated position of laser beam is moved relatively with respect to the irradiated face) thereby performing laser annealing.
Further, there is known a technology where laser beam is fabricated into a linear shape (several millimeters width)xc3x97(several tens centimeters) and irradiated while being scanned in a direction of a width of the linear beam.
When the method is used, different from the case where laser beam having a spot-like shape in which scanning in the forward and rearward direction and the left and right direction is needed, is used, the laser irradiation can be carried out on an entire irradiated face by scanning the linear laser only in a direction orthogonal to the liner direction and high productivity can be achieved.
Although the method of performing laser annealing in respect of a non single crystal semiconductor film by scanning the pulse laser beam fabricated in the above-described liner shape, is a method excellent in the productivity, several problems have been posed.
One of particularly serious problems among them is that laser annealing cannot be performed uniformly over the entire film surface.
When the linear laser began to be used, a phenomenon where stripes were formed at portions of overlapping liner beams was caused significantly and characteristics of a semiconductor considerably differed among the stripes.
FIG. 1a shows an optical photograph of a crystalline silicon film provided when the laser beam (KrF excimer laser) having a linear shape extending in the transverse direction of paper face is irradiated by scanning the beam from bottom toward top of paper face.
As apparent from FIG. 1a, stripe patterns are observed at portions where linear beams overlap. The stripe patterns reflect a difference in crystallinity in the film.
When a liquid crystal display is fabricated by using, for example, the film shown by FIG. 1a, there causes inconvenience where the stripes are shown on the screen as they are.
The reason is that a variation in the characteristic of the fabricated TFT emerges by reflecting a difference in crystallinity in the film having stripes as shown by FIG. 1a. 
It can be reduced to a nonproblematic level by devises as follows.
(1) Improvement of an non single crystal semiconductor film that is an object of irradiating laser is improved.
(2) Making a scan pitch (interval between contiguous linear laser beams) of a linear laser fine.
(3) Pursuit of an optimum combination of parameters determining various irradiating conditions. For example, a combination of parameters of scan pitch of linear laser, scan speed, pulse oscillation interval, irradiation energy density and the like is optimized.
When the above-described stripe patterns were made inconspicuous, the nonuniformity of energy distribution of the laser beam per se began conspicuous.
Generally, when the linear laser beam is formed, a beam originally having a rectangular shape is fabricated into that of the linear shape by passing the beam through pertinent lens groups.
The aspect ratio of the beam having a rectangular shape falls in a range of about 2 through 5 and the beam is deformed into a linear beam having an aspect ratio of 100 or higher by lens groups (referred to as beam homogenizer).
In this case, the lens groups are designed to homogenize simultaneously the energy distribution in the beam. According to the method of making uniform the energy distribution, the original to rectangular beam is divided and thereafter, the divided portions of the beam are respectively magnified and overlapped to homogenize the beam.
In respect of the beam which has been divided and reconstructed by such a method, apparently, the finer the division the more uniform the energy seems to be distributed.
However, when the beam is actually irradiated on a semiconductor film, despite the fineness of the division, stripe patterns as observed in FIG. 1b are formed on the film.
The stripe patterns emerge to extend in a direction of the width of the linear beam. That is, FIG. 1b shows a silicon film that is produced by irradiating a laser beam having a longitudinal direction in the left and right direction of paper face by scanning the laser beam from bottom toward top of paper face. The stripe patterns shown by FIG. 1b orthogonal to the stripe patterns shown by FIG. 1a caused by way of overlapping linear laser beams, are shown on the film.
Incidentally, although stripes in the vertical direction are observed also in FIG. 1a, in this case, the photographing conditions are set such that horizontal stripes are easy to observe and therefore, the vertical stripes do not emerge so significantly as shown by FIG. 1b. 
Further, when stripes in the vertical direction slightly observed in FIG. 1a are made easy to observe, vertical stripes as shown by FIG. 1b are observed. That is, nonuniformity of annealing (nonuniformity of crystallinity) represented by the vertical stripes in FIG. 1a and FIG. 1b are actually in the same state.
The stripe patterns in the vertical direction of FIG. 1a and FIG. 1b are formed in an innumerable number orthogonally to the longitudinal direction of the linear laser beam.
As factors of forming the vertical stripes as shown by FIG. 1b, the following two causes are conceivable.
(1) The original energy distribution of the rectangular beam is inherently provided with an energy distribution having a striped shape.
(2) The cause is derived from lens groups utilized in forming the linear laser beam.
The inventors carried out a simple experiment in order to ascertain which one of the above-described items constituted the cause. According to the experiment, an investigation was performed on how the above-described vertical stripes were changed by rotating a rectangular laser beam before the laser beam was incident on lens groups.
As a result, the vertical stripes remained unchanged. Therefore, it can be concluded that formation of the vertical stripes shown by FIG. 1b relates not to the original square beam but the lens groups.
In these lens groups, homogenizing is achieved by dividing and recombining a beam having a single wavelength and an aligned phase (phase of laser beam is aligned since laser achieves intensity by aligning phase) and accordingly, it can be explained that the stripes are interference fringes of light.
Interference of light is caused by a deviation in phases when fluxes of light each having an aligned phase and the same wavelength overlap each other with a difference in optical paths. In this case, stripe pattern is apparently observed by intensifying or weakening the fluxes of light at a certain period.
The following reasoning is conceived to study the above-describe d light interference. FIG. 3 shows behavior of interference fringes when fluxes of parallel light each having an aligned phase are made to pass through a mask where five slits 301 are opened at equal intervals.
The behavior of the interference fringes is shown by using an intensity I of light. When the five slits 301 are arranged at equal intervals, a peak of interference is caused at a region A in correspondence with the center of the slit group. Further, the interference fringes are formed centering on the peak.
This behavior is investigated by applying it to a cylindrical lens group 401 and a cylindrical lens 402 shown by FIG. 4. Incidentally, each of the cylindrical lenses is provided with a shape having a longitudinal length in the depth direction of the drawing.
Further, the number of divisions of beam caused by the cylindrical lens group 401 corresponds to the number of the slits 301 in FIG. 3.
In FIG. 4, the center A of the linear beam corresponds to the portion A at the center of the slit group of FIG. 3. That is, a peak of interference is formed at the portion A in FIG. 4.
In FIGS. 3 and 4, intensified and weakened portions of interference are periodically formed centering on the point A and the distribution reaches points B and C of the drawings.
Although such a clear intensity distribution is not shown in interference fringes formed by actual laser, the reason is that wavelengths of fluxes of laser beam are not completely aligned. Also, there is provided an influence caused by transmitting light through the lenses.
The inventors prepared an optical system shown by FIG. 5 in order to erase such interference fringes. The difference of this optical system from the optical system of FIG. 4 resides in that laser beam divided by a cylindrical lens group 501 on the incident side of beam, is fabricated into a parallel ray by a succeeding cylindrical lens 502.
Such an optical system can simply be provided by suitably selecting a distance between the preceding cylindrical lens group 401 and the succeeding cylindrical lens 402 in FIG. 4.
Such a structure was conceived to restrain occurrence of interference fringes by disposing peaks of light interference at any portions of the beam.
However, even when the beam actually fabricated by this optical system was used, the vertical stripes shown by FIG. 1b were invariably formed. That is, stripes extending in the width direction of the linear beam were observed.
However, by adopting the optical system shown by FIG. 5, the intensity of stripe (may be referred to distribution of intensity) is alleviated although the amount of alleviation is small.
It is proper to interpret that the stripe is caused when way of interference is varied delicately at respective points of the beam owing to the thicknesses of the lenses.
It is impossible to dispense with the thicknesses of the lenses and therefore, it is impossible to completely dispense with the light interference in the beam.
The system of irradiating the linear laser beam is provided with a constitution shown by, for example, FIGS. 2a and 2b. According to the constitution, a cylindrical lens group 203 for dividing the beam in the longitudinal direction of the linear beam and a cylindrical lens group 202 for dividing the beam in a direction orthogonal thereto are arranged.
Incidentally, in FIGS. 2a and 2b, a combination of the cylindrical lens group 202 and a cylindrical lens 204 and a combination of the cylindrical lens group 203 and a cylindrical lens 205 provide quite a similar operation to the laser beam.
Accordingly, it is concluded that light interference as shown by FIG. 3 is caused also in the beam width direction in the linear laser beam.
It is concluded from the above-described survey that when the optical system as shown by FIGS. 2a and 2b is adopted, as shown by FIG. 6, peaks of interference 602 (represented by circle marks) are distributed in a lattice form in the linear laser beam 601.
Generally, the width of the linear laser beam is equal to or smaller than 1 mm and points of interference in the width direction are not observed. On the other hand, in respect of the longitudinal direction, the length of the laser beam is extended over a length of 10 cm or more and therefore, points of interference in the longitudinal direction are observed. These are observed as the vertical stripes shown by FIG. 1b. 
When the single one of the linear laser beam per se is observed, the interference shown by FIG. 6 is present.
On the other hand, according to the actual laser annealing operation, the linear laser beam is irradiated to overlap successively little by little. Therefore, by scanning the laser beam, the above-described interference present in a single body of the linear laser beam also overlaps.
This seems to expedite further the nonuniformity of laser annealing caused by the interference.
That is, as shown by FIG. 7, the periodic intensity distribution of energy caused by light interference is observed in the longitudinal direction of the linear laser beam 701. (As has been described, the periodic intensity distribution of energy caused by light interference is observed also in the width direction of the linea laser beam, however, the distribution does not effect a significant influence on the present invention.)
When these irradiations overlap each other as shown by FIG. 7, the stripes are emphasized.
It is an object of the present invention disclosed in the specification to provide a technology for correcting striped nonuniformity of processing distributed in the longitudinal direction of a linear beam that is caused when the linear laser beam is irradiated while scanning the beam.
According to the present invention disclosed in the specification, when a linear laser beam having a number of interference points as shown by FIG. 6 is irradiated while scanning the beam, the influence of interference is restrained from manifesting in the irradiation effect by preventing the interference points from overlapping each other.
For example, the direction of scanning a linear laser beam of a pulse oscillation type is changed to a direction slightly oblique in respect of the conventional direction.
In this case, the irradiated position is shifted delicately at respective pulses and portions of contiguous pulses overlap each other. In this way, the interference points do not overlap each other completely and are brought into a dispersed state although the interference points partially overlap.
Further, the nonuniformity of the irradiation effect caused by interference is corrected. Specifically, formation of the stripe patterns shown by FIG. 1b is corrected.
According to one aspect of the present invention disclosed in the specification, there is provided a laser irradiating device comprising
laser beam generating means for forming a laser beam by dividing and recombining the laser beam,
means for irradiating the laser beam by scanning the laser beam in a direction having a predetermined angle xcex8 (xcex8xe2x89xa00xc2x0) in respect of a direction orthogonal to a direction of dividing the laser beam,
wherein the laser beam is of a pulse oscillation type, and
wherein pulses of the laser beam are irradiated by overlapping portions of the pulses at an irradiated region.
Further, the above-described constitution is featured in that an intensity distribution periodically varied in the direction of dividing the laser beam is present in the laser beam and the distribution is caused by the laser beam generating means.
Peaks of interference caused by an optical system can be prevented from overlapping each other by the above-described constitution.
According to another aspect of the present invention, there is provided a laser irradiating device comprising
means for irradiating a linear laser beam,
means for irradiating the laser beam by scanning the laser beam in a direction having a predetermined angle xcex8 (xcex8xe2x89xa00xc2x0) in respect of a direction of a width of the liner laser beam,
wherein the laser beam is provided with an intensity distribution periodically varied in a longitudinal direction of the laser beam;
wherein the laser beam is of a pulse oscillation type, and
wherein pulses of the laser beam are irradiated by overlapping portions of the pulses at an irradiated region.
According to another aspect of the present invention, there is provided a laser irradiating device comprising
means for irradiating a linear laser beam,
means for irradiating the laser beam by scanning the laser beam in a direction having a predetermined angle xcex8 (xcex8xe2x89xa00xc2x0) in respect of a direction of a width of the linear laser beam,
wherein the laser beam is provided with an intensity distribution periodically varied in a longitudinal direction of the laser beam,
wherein the laser beam is of a pulse oscillation type,
wherein pulses of the laser beam are irradiated by overlapping portions of the pulses at an irradiated region, and
wherein the angel xcex8 is selected from a range where peaks of the periodic intensity distribution do not overlap.
The above-described constitution is featured in that the angle xcex8 is selected from a range satisfying 0.01xe2x89xa6|tan xcex8|xe2x89xa60.3.
According to another aspect of the present invention, there is provided a laser irradiating method which is a method of irradiating a linear laser beam of a pulse oscillation type having an intensity distribution periodically varied in a longitudinal direction of the laser beam, the method comprising the steps of,
scanning the laser beam in a direction having a predetermined angle xcex8 (xcex8xe2x89xa00xc2x0) in respect of a direction of a width of the liner laser beam, and irradiating pulses of the laser beam by overlapping portions of the pulses at an irradiated region.
Further, the above-described constitution is featured in that the angle xcex8 is selected from a range where peaks of the periodic intensity distribution do not overlap each other.
Further, the above-described constitution is featured in that xcex8 is selected from a range satisfying 0.01xe2x89xa6|tan xcex8|xe2x89xa60.3.
Further, the effect of the present invention is not achieved when the linear laser beam do not overlap at the irradiated face.
In this case, even when the beams do not overlap, only the presence of the interference points is revealed and therefore, even when the laser beam is scanned, the interference points are only extended in the scanning direction.
In irradiating the linear laser beam, by scanning the beam in the direction having a predetermined angle in respect of the direction orthogonal to the longitudinal direction of the beam, the periodic iteration of the intensity of the irradiation energy density that is observed in the longitudinal direction of the linear laser beam can be prevented from overlapping each other completely and interference fringes can be prevented from emerging.
That is, by changing the scanning direction of the linear laser beam, the iteration of the maximum portion or the minimum portion of energy in the beam is prevented from completely disposing at the same portion of the semiconductor film. (That is, irradiation is performed by shifting the beam little by little.)
In this way, the energy distribution in the linear laser beam is dispersed in the semiconductor film and the laser annealing can be performed more uniformly.